I"m working on a little project and would really appreciate some help from the electrical gurus. I'm trying to use a relay and resistor to make a dimming relay for daytime running lights. The DRL's will be connected to the parking lights and I would like them to dim when I turn on the headlights.

 

Using an SPDT, here's how you'd wire it up.

87a - DRL full power output
30 - DRL power input (parking lights +12v in your case)
87 - DRL DIMMED output
85 - GND
86 - Switched +12v (in your case, +12v from the headlight switch)

Terminal 30 is the common terminal, and this will have power whenever the parking lights are on.
Terminal 85 is the ground side of the coil.
Terminal 86 is the +12v side.
Terminal 87a is normally closed, so you'll have power going through this terminal at all times that terminal 30 has power.
Terminal 87 is normally open, and will close when you turn the headlights on.

 

Forgot to mention: If the relay you're using has a diode, you might need to swap terminals 85 and 86. I haven't done this in quite some time so I don't remember which side is supposed to positive when there's a diode in there. Oh, and the resistor would go on terminal 87.

 

Wow, first you set me up with my Volks at an awesome price, and now this. Vivid Racing FTW! Seriously, just what I was looking for, thanks a million!

 

You need to know the intended polarity of the relay coil BEFORE you apply power to it. If you hook it up with the wrong polarity, it fries the diode.

The purpose of the diode is to stop the reverse voltage spike (high voltage, low amperage) that happens when you remove power from the coil.

 

Thanks for the kind words! I'm glad we could help. Shoot me a PM if you need more help.